GVB
Information about the Amsterdam bus, tram and metro services including night buses, day tickets, the Strippenkaart, where to buy tickets and how to get to major tourist attractions in the city using public transport.
http://www.gvb.nl/touristguide/index-en.htm

Bookings.nl
Unofficial but comprehensive guide to the city's bus service with bus routes, times, station details and more.
http://www.apti.is.nl/busline.html

Tickets, please - Hotel Prinsenhof
A guide to using the Strippenkaart, which allows you to take multiple rides on Amsterdam's buses, trams and metro system.
http://www.hotelprinsenhof.com/mokum/gvb.html